1. Field of the Invention
The present invention relates to a semiconductor substrate and a method of fabricating a semiconductor device and, more particularly, to a semiconductor substrate and a method of fabricating a semiconductor device which prevent particles of dust from being produced at an edge of the substrate.
2. Description of the Background Art
An SOI (silicon on insulator) device including a semiconductor element formed on an SOI substrate is superior to a bulk device in its decreased junction capacitance and improved device isolation breakdown voltage, but has inherent problems to be described below.
FIG. 40 is a sectional view of an SOI substrate 10. The SOI substrate 10 has a triple layer structure comprising a silicon substrate 1, a buried oxide film 2 formed in an upper major surface of the silicon substrate 1, and a single crystalline silicon layer (referred to hereinafter as an SOI layer) 3 formed on the buried oxide film 2. A polysilicon layer 4 is formed on edges and a lower major surface of the single crystalline silicon substrate 1. The polysilicon layer 4 is provided to getter contaminants such as heavy metal provided during wafer fabrication steps and a transistor wafer process. A structure having such a polysilicon layer is known as a poly-back-coat structure (PBC structure).
Methods of fabricating the SOI substrate include a SIMOX (separation by implanted oxygen) method and a bonding method. The SOI substrate fabricated by the SIMOX method (SIMOX substrate) is employed as an example in the following description.
In the SIMOX method, oxygen ions are implanted into a single crystalline silicon substrate at a dose of, for example, 0.4xc3x971018/cm2 to 3xc3x971018/cm2, and thereafter the silicon substrate is annealed at a temperature of about 1350xc2x0 C. to provide the SOI structure.
FIG. 41 is a partial detailed view of an edge of the SOI substrate 10. For purposes of explanation, the semiconductor substrate is divided into four sections: an upper major surface (on which semiconductor elements are to be formed), a central section of the upper major surface (including active regions), an edge section including a section surrounding the central section and side surfaces, and a lower major surface.
FIG. 41 shows an area X in detail in which the buried oxide film 2 and the SOI layer 3 meet the polysilicon layer 4. As illustrated in FIG. 41, since the edge section has a curved surface having a great curvature, vertically directed oxygen ions are implanted in a slanting direction into the edge section, decreasing an effective implantation energy in the edge section. The result is the reduction in the thickness of the buried oxide film 2 and the SOI layer 3 in the edge section, creating a structure wherein the SOI layer 3 is prone to exfoliate.
Additionally, the step of thinning the SOI layer 3 during the fabrication of the SOI device promotes the exfoliation of the SOI layer 3. The step of thinning the SOI layer 3 is described with reference to FIGS. 42 and 43.
The SOI layer 3 in the SOI substrate 10 has a suitable thickness as shown in FIG. 42 when the substrate is fabricated. The step of thinning the SOI layer 3 is to suitably reduce the thickness of the SOI layer 3 in accordance with the specs of a desired semiconductor device, and comprises oxidizing the SO1 layer 3 and removing the resultant oxide film to adjust the thickness of the SOI layer 3.
FIG. 43 shows an oxide film 5 formed on the SOI layer 3. The thickness of the oxide film 5 is generally determined based on the thickness of the SOI layer 3 in the central section of the SOI substrate 10, that is, semiconductor element formation regions (active regions). The problems that arise herein are the reduced thickness of the SOI layer 3 in the edge section of the SOI substrate 10 as above described, and the formation of the polysilicon layer 4 in the edge section of the SOI substrate 10. An area Y shown in FIG. 42 is illustrated in more detail in FIG. 44, and an area Z shown in FIG. 43 is illustrated in more detail in FIG. 45. FIG. 46 shows the edge section after the removal of the oxide film 5.
As illustrated in FIG. 44, the polysilicon layer 4 is comprised of a multiplicity of single crystal grains GP. Because of individually different crystal orientations of the single crystal grains GP, the oxygen ions are implanted to different depths due to channeling, causing the buried oxide film 2 to be formed at varied depths.
Further, different oxidation rates of the polysilicon layer 4 depending on the crystal orientations of the single crystal grains GP result in different thicknesses of the oxide film 5 in accordance with the respective single crystal grains GP as shown in FIG. 45 after the oxidation of the polysilicon layer 4.
The reduced thickness of the SOI layer 3 in the edge section of the SOI substrate 10 might cause the oxide film 5 to be contact with the buried oxide film 2 depending on the single crystal grains GP and cause the SOI layer 3 to be completely oxidized. In such cases, part of the SOI layer 3 might be surrounded by the buried oxide film 2 and the oxide film 5. For example, an SOI layer 30 shown in FIG. 45 is surrounded by the oxide film 5 and the buried oxide film 2.
When wet etching is performed on the oxide film 5 using an etchant such as hydrofluoric acid for thinning the SOI layer 3 in the SOI substrate 10 under the above described conditions, the buried oxide film 2 as well as the oxide film 5 is etched as shown in FIG. 46. Then, the SOI layer 30 is lifted off into a particle suspended in the etchant. In some cases, the SOI layer 30 might adhere to the central section of the SOI substrate 10. The adhesion of particles to the semiconductor element formation regions causes the formation failures of semiconductor elements and, accordingly, the decrease in fabrication yield.
As above described, the background art semiconductor substrate, particularly the SOI substrate, has the drawback that the SOI layer in the edge section of the substrate exfoliates into particles to cause the decrease in fabrication yield. The production of the particles is also a problem for semiconductor substrates other than the SOI substrate.
A first aspect of the present invention is intended for a method of fabricating a semiconductor device using a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section. According to the present invention, the method comprises the steps of: (a) forming a first oxide film so as to cover the central section and the edge section of the semiconductor substrate; (b) selectively forming an oxidation-resistant film on the first oxide film in the central section; (c) further oxidizing the edge section of the semiconductor substrate using the oxidation-resistant film as a mask to form a second oxide film in the edge section, the second oxide film being thicker than the first oxide film; and (d) forming semiconductor elements in the active regions.
Preferably, according to a second aspect of the present invention, in the method of the first aspect, the semiconductor substrate is an SOI substrate formed by a SIMOX technique; the semiconductor substrate comprises a buried oxide film and an SOI layer formed in a sequentially stacked relation in the entire first major surface; and the step (c) comprises the step of (c-1) forming the second oxide film so as to completely oxidize the SOI layer extending in the edge section and to oxidize part of the edge section which has not been oxidized.
Preferably, according to a third aspect of the present invention, in the method of the first aspect, the semiconductor substrate is an SOI substrate formed by a bonding technique; the semiconductor substrate comprises an on-substrate oxide film and an SOI layer formed in a sequentially stacked relation on the entire first major surface; and the step (c) comprises the step of (c-1) forming the second oxide film so as to completely oxidize the SOI layer extending in the edge section and to oxidize part of the edge section which has not been oxidized.
Preferably, according to a fourth aspect of the present invention, in the method of the first aspect, the semiconductor substrate is a bulk silicon substrate; the semiconductor substrate comprises a polysilicon layer formed on the edge section and the second major surface; and the step (c) comprises the step of (c-1) forming the second oxide film so that the polysilicon layer is not completely oxidized.
Preferably, according to a fifth aspect of the present invention, in the method of the second aspect, the step (a) comprises the step of forming the first oxide film so that the thickness of the SOI layer in the central section is reduced to a thickness conforming to formation of semiconductor elements.
Preferably, according to a sixth aspect of the present invention, in the method of the fifth aspect, the step (b) comprises the step of forming a pattern of the oxidation-resistant film in accordance with the pattern of a field oxide film defining the active regions in the central section; and the step (c) comprises the step of forming the second oxide film as the field oxide film in accordance with the pattern of the oxidation-resistant film in the central section.
A seventh aspect of the present invention is intended for a method of fabricating a semiconductor device using a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section. According to the present invention, the method comprises the steps of: (a) forming an oxide film so as to cover the central section and the edge section of the semiconductor substrate; (b) forming a resist mask on the oxide film except in the central section; (c) selectively removing the oxide film in the central section using the resist mask as an etching mask to expose the semiconductor substrate, with the oxide film left in the edge section; and (d) forming semiconductor elements in the active regions.
Preferably, according to an eighth aspect of the present invention, the method of the seventh aspect further comprises the step of (e) forming an oxidation-resistant film on the oxide film in the edge section.
Preferably, according to a ninth aspect of the present invention, in the method of the seventh aspect, the semiconductor substrate is an SOI substrate formed by a SIMOX technique; the semiconductor substrate comprises a buried oxide film and an SOI layer formed in a sequentially stacked relation in the entire first major surface; and the step (a) comprises the step of forming the oxide film so that the thickness of the SOI layer in the central section is reduced to a thickness conforming to formation of semiconductor elements.
A tenth aspect of the present invention is intended for a method of fabricating a semiconductor device using a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section, the semiconductor substrate being an SOI substrate formed by a SIMOX technique, the semiconductor substrate including a buried oxide film and an SOI layer formed in a sequentially stacked relation in the entire first major surface. According to the present invention, the method comprises the steps of: (a) forming a first oxide film so as to cover the central section and the edge section of the semiconductor substrate; (b) selectively forming a resist mask on the first oxide film in the central section; (c) selectively removing the first oxide film and the SOI layer in the edge section of the semiconductor substrate using the resist mask as an etching mask to expose the buried oxide film; (d) further oxidizing the first oxide film under the resist mask to form a second oxide film thicker than the first oxide film and to increase the thickness of the buried oxide film exposed; and (e) forming semiconductor elements in the active regions.
An eleventh aspect of the present invention is intended for a method of fabricating a semiconductor device using a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section, the semiconductor substrate being an SOI substrate formed by a SIMOX technique, the semiconductor substrate including a buried oxide film and an SOI layer formed in a sequentially stacked relation in the entire first major surface. According to the present invention, the method comprises the steps of: (a) forming a first oxide film so as to cover the central section and the edge section of the semiconductor substrate; (b) selectively forming a resist mask on the first oxide film in the central section; (c) selectively removing the first oxide film, the SOI layer and the buried oxide film in the edge section of the semiconductor substrate by dry etching using the resist mask as an etching mask to expose an underlying substrate under the SOI layer; (d) further oxidizing the first oxide film under the resist mask to form a second oxide film thicker than the first oxide film and to form a third oxide film on the underlying substrate exposed; and (e) forming semiconductor elements in the active regions.
Preferably, according to a twelfth aspect of the present invention, in the method of the tenth aspect, the step (d) comprises the step of forming the second oxide film so that the thickness of the SOI layer in the central section is reduced to a thickness conforming to formation of semiconductor elements.
A thirteenth aspect of the present invention is intended for a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section. According to the present invention, the semiconductor substrate comprises: a buried oxide film and an SOI layer formed in a sequentially stacked relation in the entire first major surface; and an oxide film formed in the edge section and having a thickness reaching the buried oxide film.
A fourteenth aspect of the present invention is intended for a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section. According to the present invention, the semiconductor substrate comprises: a buried oxide film and an SOI layer formed in a sequentially stacked relation in the first major surface, wherein the buried oxide film contains silicon islands, and wherein the density of the silicon islands is lower in the buried oxide film extending in the edge section than in the buried oxide film in the central section.
A fifteenth aspect of the present invention is intended for a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section. According to the present invention, the semiconductor substrate comprises: a buried oxide film and an SOI layer formed in a sequentially stacked relation in the first major surface, wherein the buried oxide film contains silicon islands, and wherein the buried oxide film and the SOI layer are not formed in the edge section.
In accordance with the method of the first aspect of the present invention, the relatively thick second oxide film is formed on the edge section. If a layer which is prone to exfoliate due to wet etching is present on the edge section and second major surface, the second oxide film functions as a protective film to eliminate the problem that part of the layer which is prone to exfoliate is lifted off into particles suspending in the etchant. This prevents the formation failures of the semiconductor elements resulting from the presence of the particles, increasing the fabrication yield.
In accordance with the method of the second aspect of the present invention, the second oxide film is formed on the edge section of the SOI substrate formed by the SIMOX technique so that the SOI layer in the edge section is completely oxidized and the part of the edge section which has not been oxidized is oxidized. Thus, the SOI layer which is prone to exfoliate due to wet etching is protected, and eliminated is the problem that part of the SOI layer is lifted off into particles suspending in the etchant. This prevents the formation failures of the semiconductor elements resulting from the presence of the particles, increasing the fabrication yield.
In accordance with the method of the third aspect of the present invention, the second oxide film is formed on the edge section of the SOI substrate formed by the bonding technique so that the SOI layer in the edge section is completely oxidized and the part of the edge section which has not been oxidized is oxidized. If the edge section of the on-substrate oxide film and SOI layer is not perfectly chamfered to provide a continuously uneven or rough peripheral configuration in plan view, the uneven or rough periphery is prevented from exfoliating into particles, and the edge section of the on-substrate oxide film is prevented from being partially removed during wet etching.
In accordance with the method of the fourth aspect of the present invention, the bulk silicon substrate includes the polysilicon layer formed on the edge section and second major surface. In this case, the presence of the second oxide film formed on the edge section so that the polysilicon layer is not completely oxidized prevents the polysilicon layer from exfoliating during wet etching due to a structure inherent in the polysilicon layer.
In accordance with the method of the fifth aspect of the present invention, the thickness of the first oxide film is made suitable for thinning the SOI layer. This eliminates the need to reduce the thickness of the SOI layer in a subsequent step, simplifying the steps of processing the semiconductor substrate.
The method of the sixth aspect of the present invention is capable of forming the second oxide film and the field oxide film at the same time, simplifying the steps of processing the semiconductor substrate.
The method of the seventh aspect of the present invention is capable of readily and conveniently forming the oxide film on the edge section of the semiconductor substrate to significantly simplify the steps of processing the semiconductor substrate, reducing processing costs.
In accordance with the method of the eighth aspect of the present invention, the edge section of the semiconductor substrate is rigidly protected by the oxide film and the first oxidation-resistant film.
In accordance with the method of the ninth aspect of the present invention, the thickness of the oxide film is made suitable for thinning the SOI layer. This eliminates the need to reduce the thickness of the SOI layer in a subsequent step, simplifying the steps of processing the semiconductor substrate.
In accordance with the method of the tenth aspect of the present invention, the buried oxide film exposed in the edge section of the SOI substrate is exposed to oxygen serving as an oxidizing agent in the step (d). Thus, as the oxygen is diffused in the buried oxide film to reach silicon islands inherent in the buried oxide film of the SOI substrate formed by the SIMOX technique, the oxygen reacts with silicon to form a silicon oxide film, resulting in disappearance of the silicon islands. The result is the reduced number of silicon islands in the buried oxide film in the edge section of the SOI substrate. If the buried oxide film is removed by wet etching, the silicon islands are prevented from being lifted off into particles.
In accordance with the method of the eleventh aspect of the present invention, dry etching is used to selectively remove the first oxide film, the SOI layer, and the buried oxide film in the edge section of the semiconductor substrate. This permits the silicon islands inherent in the buried oxide film of the SOI substrate formed by the SIMOX technique to disappear in the edge section of the semiconductor substrate, preventing the silicon islands from being lifted off into particles during wet etching.
In accordance with the method of the twelfth aspect of the present invention, the thickness of the second oxide film is made suitable for thinning the SOI layer. This eliminates the need to reduce the thickness of the SOI layer in a subsequent step, simplifying the steps of processing the semiconductor substrate.
In accordance with the semiconductor substrate of the thirteenth aspect of the present invention, the presence of the oxide film formed on the edge section of the semiconductor substrate and having the thickness reaching the buried oxide film may protect the SOI layer which is prone to exfoliate due to wet etching to eliminate the problem that part of the SOI layer is lifted off into particles suspending in the etchant. This prevents the formation failures of the semiconductor elements resulting from the presence of the particles, providing semiconductor substrates with an increased fabrication yield.
In accordance with the semiconductor substrate of the fourteenth aspect of the present invention, the density of the silicon islands is lower in the buried oxide film in the edge section of the semiconductor substrate than in the buried oxide film in the central section of the first major surface. If the buried oxide film is removed by wet etching, the semiconductor substrate which prevents the silicon islands from being lifted off into particles is accomplished.
In accordance with the semiconductor substrate of the fifteenth aspect of the present invention, the buried oxide film and the SOI layer are not formed in the edge section of the semiconductor substrate. This accomplishes the semiconductor substrate which prevents the silicon islands from being lifted off into particles during wet etching.
It is therefore an object of the present invention to provide a semiconductor substrate and a method of fabricating a semiconductor device which prevent particles of dust from being produced at an edge of the substrate.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.